1. Field of the Invention
The present invention relates to insensitive crystalline high explosive molding powders, and more particularly to a spray drying method for producing insensitive explosive molding powders where the crystallization and coating of the high explosive occur in a single step.
2. Related Art
Sensitivity of munitions to undesired stimuli, such as shock and impact, increases the potential of accidental initiation, which can result in loss of life, as well as, significant cost and compromised capabilities. Minimizing such sensitivity is therefore highly desired.
A particularly critical application which involves balancing insensitivity and explosive effectiveness involves booster explosives, which must have a sufficient energy output to reliably initiate newer, relatively insensitive main charge explosive fills, while having themselves a lower level of sensitivity to unintended stimuli. Most existing booster high explosive (HE) formulations exhibit unacceptable levels of sensitivity thereby increasing the vulnerability of the entire munition to accidental initiation.
It is well known that the crystal size of a HE can significantly influence its sensitivity to unintended stimuli such as shock and impact; specifically, it has been demonstrated that the sensitivity of a high explosive decreases with decreasing crystal size. See, Stepanov et al. “Processing and Characterization of Nanocrystalline RDX”, Proceedings of the 39th International Annual Conference of ICT, 2008 Karlsruhe, Germany. Further, improved performance characteristics are also associated with crystal size reduction. For example, the detonation failure diameter, also referred to as the critical diameter, is known to decrease with decreasing crystal size.
There are two relatively complex methods known to produce basically pure, nanocrystalline HE, including 1,3,5-trinitro-1,3,5-triazacyclohexane, also known as RDX. The first method produces RDX with a mean crystal size in the range from around 100 to 500 nm is disclosed in Stepanov et al, “Production of Nanocrystalline RDX by Rapid Expansion of Supercritical Solutions”, Propellants, Explosives, Pyrotechnics Vol. 30, No. 3, pages 178-183 (Wiley-VCH Verlag, GmbH & Co., KGaA, Weinheim, 2005). The second method which uses a bead mill to produce RDX with a mean crystal size below 500 nm is disclosed in R. Patel et al., “Slurry Coating Process for Nano-RDX produced by a Bead Mill”, NDIA Insensitive Munitions and Energetic Materials Technology Symposium, 2008, Miami, Fla.
An alternative method that produces a broad range of pure RDX crystals from 400 nanometer particles to several micron particles, involves the evaporative crystallization of RDX by spray drying an RDX/acetone solution. See, Van der Heijden et al., “Energetic Materials: Crystallization, Characterization, and Insensitive Plastic Bonded Explosives, Propellants, Explosives, Pyrotechnics, Vol. 33, No. 1, pages 25-32 (Wiley-VCH Verlag, GmbH & Co., KGaA, Weinheim, 2008).
U.S. Pat. No. 6,485,587, issued Nov. 26, 2002 to Han et al., incorporated herein by reference, discloses traditional methods used for the preparation of explosive molding powders typically consist of batch slurry coating of crystalline HE with a binder. In these processes, the explosive crystals are dispersed in an aqueous slurry, to which a lacquer solution consisting of an organic solvent and the binder ingredients are added. Dispersion of nano-crystals in an aqueous slurry is not effective due to the high tendency of the such small crystals to agglomerate, resulting in poor coating of the crystals.
Handling of uncoated HE nanoparticles, such as occurs in the production of the nanocrystalline HE, and the subsequent processing, poses a health hazard. Such small particles are easily airborne and absorbed into the body.
There is a need in the art for a relatively insensitive HE, with good performance characteristics, that is manufactured in a safe, relatively simple and economical way.